Microvascular endothelial cells in the brain have a number of special properties that underlie formation of the blood-brain barrier (BBB) and contribute to control of the neuronal microenvironment. Evidence from transplantation experiments indicates that signals arising within brain rather than a programmed commitment of the endothelial cells are responsible for the expression of blood-brain barrier properties. The close anatomic relationship between brain endothelial cells and the foot processes of astrocytes suggests a role for astrocytes as a source of the differentiation signals. It is now possible to isolate and separately culture populations of brain-derived endothelial cells and astrocytes. When the two cell types are grown together, a characteristic morphologic organization occurs that is associated with induction of enzymes and tight junctions similar to those found in vivo. Endothelial cells and astrocytes in culture differ in their production of and response to specific polypeptide growth factors. These findings provide the basis for a model of endothelial cell-astrocyte interaction that may explain several aspects of BBB behavior.